The present invention relates generally to the field of explosives and more particularly to means, known as detonators, used to set off or detonate explosives. More particularly, the present invention relates to a capacitor exploding foil initiator device.
While explosives have long been used in various fields, their use is always subject to the dangers of premature explosion with resultant injury and damage. Explosives that produce the greatest force are likely to be explosives that are not easily detonated by heat or shock. To achieve the desired safety it is advisable to use explosives that are not likely to be unintentionally detonated by a rise in temperature or by a moderate shock. While such explosives are known, the characteristics that contribute to their safety also contribute to the difficulty of detonating them at the desired time. Thus, the detonation of such an explosive requires the use of some other explosive trigger means and the trigger means, in turn, must be one that is not subject to premature explosion. Many of these problems can be solved by the use of a less sensitive explosive such as HNS, (hexanitrostilbene) that, in turn, is detonated by means such as an exploding bridge.
It has long been known that the passage of an electric current through a conductor generates a certain amount of heat, the amount of heat varying directly with the resistance of the conductor and with the square of the current. This phenomenon is relied upon in fusible links that are installed in electrical circuits to prevent the flow of more than a predetermined amount of current in such a circuit. When the predetermined flow is exceeded, the heat melts the fusible link so that the circuit is broken. If a sufficient current is passed through the link in a small period of time, the link is not only melted but may be vaporized. If the fusible link is enclosed in a small space the vaporizing of the link can increase the pressure in that space.
For a number of years it has been customary to detonate an explosive by means of a blasting cap having a heat sensitive explosive set off by an electrical resistance heated by the passage of an electric current through the resistance. More recently, explosives have been detonated by means making use of a relatively high resistance bridge extending between conductors and through which a relatively high current is passed so that the bridge portion is not only heated to its melting point but is heated so much that it vaporizes and literally explodes to provide a shock wave to detonate the explosive. While such a system can use an explosive that is much less sensitive to heat and shock, there are still a distressing number of accidents that occur when an explosive is prematurely detonated. While less sensitive explosives have heretofore been available, it has been difficult to cause the detonation of such explosives at a selected time.
Recently, it has been proposed to detonate these more stable explosives by an electrical means of some sort that creates a sudden pressure to shear a film and form a disk or flyer which is then impacted against the explosive material.
In the construction of such a detonator, it is important that the explosive material be properly supported and sealed against the admission of materials such as moisture that would tend to deteriorate it. This is particularly important when the detonator is used in environments, such as deep wells, where the ambient pressures can become very high. It is also important that the physical construction be such that the flyer has sufficient kinetic energy imparted to it to insure the detonation of the explosive.
One such example is disclosed in U.S. Pat. No. 4,602,565 (which is incorporated herein by reference) wherein a detonator for a primary explosive uses a less sensitive secondary explosive that in turn is detonated by a flyer that is sheared from a sheet or film and propelled through a barrel to impact the secondary explosive. The flyer is sheared from the sheet by the pressure generated when an electrical conductor adjacent the sheet is vaporized by the sudden passage of a high current (as by the discharge of a capacitor) through it. The explosive is sealed against moisture, and the mechanical configuration of the detonator is such as to take full advantage of the kinetic energy of the flyer.
As is typical in the prior art, the capacitor is in a circuit with the foil detonator and a normally open switch. When it is desired to arm the system, the capacitor is charged, e.g., to 3000 volts; when it is desired to initiate the explosion, the switch is closed and the capacitor discharges through the foil vaporizing the same. A high resistance bleed resistor connected across the capacitor is used to bleed off the charge on the capacitor in the event that the latter is charged but then not discharged into the load.